It's the cause of death for nearly 1.5 million people each year mostly in developing countries. Tuberculosis (TB) is a lung disease that is caused by bacteria. The symptoms include fever, bloody cough and fatigue. World Tuberculosis Day is observed March 24 to "raise awareness about the burden of tuberculosis (TB) worldwide and the status of TB prevention and control efforts," according to the World Health Organization. Here are five things you didn't know about tuberculosis.

Tuberculosis Is Caused by Bacteria and Spread Through the Air

According to the World Health Organization, "when people with lung TB cough, sneeze or spit, they propel the TB germs into the air. A person needs to inhale only a few of these germs to become infected."

Tuberculosis Is Growing More Resistant to Treatment Worldwide

Tuberculosis is growing more resistant to treatment worldwide, according to a study released in August 2012 in the journal The Lancet, a finding that suggests the potentially fatal disease is becoming more difficult and costly to treat.

Although it is curable, the treatment regimen requires patients dutifully to take multiple antibiotics daily for several months, and if there are any deviations from protocol or incomplete courses, drug resistance develops easily.

The WHO estimates that about 5 percent of the cases of this disease are multidrug-resistant tuberculosis, or MDR-TB. In other words, they are caused by bacteria that have developed resistance to two of the first-line tuberculosis drugs, isoniazid and rifampicin.

Worse, as additional antibiotics are being thrown at the disease, forms that are even more resistant have begun to emerge. First reported in 2006, cases of extensively drug-resistant tuberculosis (XDR-TB) are resistant to drugs called fluoroquinolones, as well as to one of the three available intravenous drugs. While MDR-TB is difficult and costly to treat, XDR-TB is even harder.

Tuberculosis Is Global Threat

Tuberculosis kills at least 1.34 million people each year worldwide. And now the disease, once curable with antibiotics, is becoming resistant to multiple drugs.

Although most cases of multidrug-resistant TB are in developing countries, there were 92 U.S. cases reported in 2011, according to WHO data.

People With Weak Immune Systems Are More Susceptible to Getting TB

People with weak immune systems or those who have HIV are at greater risk in contracting TB. It is the leading killer of people with HIV, according to WHO.

Also, smoking and tobacco use increases the risk of TB. According to WHO, more than 20 percent of TB cases globally are attributable to smoking.

Drug-Resistant TB Could Bring Back Sanatoria

Like other superbugs, antibiotic-resistant tuberculosis does not succumb to typical treatments. And, left untreated, infected patients can readily pass the sinister strain of bacteria onto family members and others in their communities.

Drug-resistant tuberculosis is not a public health problem in the United States, thanks in part to the strict supervision of at-home tuberculosis treatment.

But in regions of the world increasingly burdened by drug-resistant tuberculosis, the idea of sanatoria might be worth revisiting.

Before the advent of antibiotics, people with infectious diseases like tuberculosis were sent to sanatoria, secluded hospitals that healed through good food, fresh air and sunlight. The isolated buildings also quarantined infected patients, thwarting the spread of contagious and dangerous diseases.

In the '60s, rifampicin removed the need for sprawling sanatoria, some of which, like the Sondalo Tuberculosis Hospital in Italy, housed several thousand patients. But the recent emergence of new, antibiotic-resistant strains of the disease-causing bacteria in South Africa has prompted a call for the return of sanitoria.